The present invention relates to measurements of partially light-transmissive thin films or layers of such films, and to measurements of surface topography or detection of surface defects. It specifically relates to optical measurement or detection, and to apparatuses for performing such optical measurement or detection of a thin film or a substrate surface.
A number of common articles of manufacture now have constructions involving thin films formed on relatively large area smooth substrates, and substrates wherein the underlying surface is reflective, possibly conductive, and at least visually smooth if not optically flat. To develop manufacturing processes for reliably fabricating these articles and to inspect them or understand the defects which arise in these articles, it is necessary to observe the thin films and the underlying substrate. These films may be liquid or solid, have a thickness substantially under one wavelength of the observation illumination, and may possess features or defects which are observable only with meticulous methodology against the highly reflective substrate, requiring a special instrument. To detect changes occurring on such a thin surface coating is an even more challenging task.
Various optical diagnostic methods, such as reflection ellipsometry, have been proposed to study thin film layers and surfaces. Reflection ellipsometry is the measurement of the change in polarization of light upon reflection from a specular surface to obtain information about the surface. Conventional automatic ellipsometers employ a rotating optical element, usually a rotating analyzer, to measure the polarization of the specularly reflected light beam. A significant drawback of these ellipsometers is that the instruments are relatively slow and thus are not suitable for real-time analysis.
A somewhat faster ellipsometer, a polarization-modulated ellipsometer (PME), is described in a paper of Jellison and Modine (Applied Optics, Vol. 29, No. 7, pg. 959 (March 1990)). This ellipsometer employs a photo-elastic modulator that dynamically elliptically polarizes the light incident on the sample surface and separates the analyzed light into orthogonally polarized beams using a Wollaston prism. The time resolution of this system is limited by the modulation frequency of the phase modulator which is approximately 50 kHz. The optimal time resolution of this type of ellipsometer is described as 10-ms, which remains impractical for real-time or in-situ analysis during processing or, in the case of magnetic storage disks, during use.
As the above described and other prior art devices and methods for performing optical measurement or detection of a thin film or a substrate surface have proven less than optimal, it is an object of the present invention to provide nondestructive diagnostic systems and methods having improved sensitivity, speed, and time resolution.
Another object of the present invention is to provide optical measurement systems and methods in which the surface of a substrate can be analyzed by a single optical scan of the substrate surface.
A further object of the present invention is to provide optical measurement systems and methods for real-time and in situ measurement and detection of changes or defects in a thin film layer and the underlying substrate surface.
Other general and more specific objects of this invention will in part be obvious and will in part be evident from the drawings and the description which follow.